A conventional mower moves at random within a closed range to remove the weeds within the range. A continuous conductive wire, arranged along an outer border and an inner border of the closed range, is conducted to generate a magnetic field after current pass through the continuous conductive wire. When the mower moves to a border, the mower determines the border of the closed range by sensing the magnetic field, and then returns to the closed range to continue mowing the weeds at random. However, when the continuous conductive wire is cut off by the mower and ends up with power failure, the continuous conductive wire is no longer able to generate the magnetic field. Under such circumstance, the mower is unable to determine the border of the closed range, and may move outside the closed range.